The small size of the Mac mini makes it a great file server. However, the built-in FTP abilities on a Mac will leave a little to be desired. (For Example, there is no anonymous FTP ability.)
However, there is a great application called PureFTPd Manager that is available. With this app, you can set up many accounts assigned only to specific folders. You can also have anonymous FTP clients. This application is free.
With this great application, the Mac mini is a great FTP Server.